Energy storage and renewables

BIOENERGY

Hold on - is burning biomass

bad for the climate?

Is replacing coal burning with biomass good for climate change or, as some have suggested,

does it actually create more emissions? John D Sterman argues the latter, based on a

recently published lifecycle analysis of US forests.

greenhouse gas causing global

century at least - even when wood

warming.

displaces coal, the most carbon-

intensive fuel.

• Burning wood adds carbon

dioxide to the atmosphere.

Repaying carbon debts

• Carbon dioxide is only removed

Although wood has approximately

from the atmosphere if the

the same carbon intensity as

forests harvested to supply

coal per kwh, the combustion

wood bioenergy grow back and

and processing efficiencies of

keep that carbon sequestered in

wood pellets, the dominant wood

biomass and soils.

bioenergy in the EU and the UK,

are lower. Consequently, pellet-

• Regrowth takes time.

fired power plants generate more

• Regrowth is not certain. Fire,

carbon dioxide per kWh than coal.

insect damage, re-harvest, or

Burning wood instead of coal

ood bioenergy is booming

conversion to other uses (eg

therefore creates a carbon debt - an

as governments around

agriculture, development) can

immediate increase in atmospheric

the world struggle to cut

limit or prevent forest recovery.

carbon dioxide compared to fossil

their greenhouse gas emissions.

energy.

The European Union declared all

The climate impact of wood and

That carbon debt can be repaid

biofuels to be carbon neutral to help

other biofuels therefore depends on

over time - if the forest regrows. To

meet its goal of 20% renewable

some critical questions.

illustrate, Figure 1 depicts a

energy by 2020, triggering a surge

First, at the point of combustion,

simulation showing what happens

in wood use. The UK subsidises

do biofuels generate more or less

to the carbon stored in a hardwood

wood pellets for electric power

carbon dioxide per unit of end-use

forest in the southern US after

generation and is now the world’s

energy than the fuel they displace?

clear-cut (where all trees are cut

largest pellet importer. The US has

Emissions accounting must consider

down).

just declared forest bioenergy to be

emissions both from combustion

Clearcutting removes nearly all

carbon neutral, while at the 2017

and from the fuel supply chain.

the biomass, burning it for

UN climate summit 19 nations

Second, how much carbon can

bioenergy adds carbon dioxide to

including Brazil, Canada, China,

the harvested land remove from the

the atmosphere, accelerating

India and Indonesia joined the

atmosphere as it grows back, how

climate change. Lacking sufficient

Biofuture Platform initiative to

long does regrowth take, and how

biomass to move carbon from leaves

increase use of ‘sustainable

do the answers to these questions

to roots, the stock of carbon stored

bioenergy’.

depend on the fate of the harvested

in the soil falls for several decades

But are biofuels really carbon

land?

even as trees start to grow back.

neutral? Is wood really good?

To answer these questions my

Biomass and soil carbon gradually

The appeal is intuitive: burning

colleagues and I extended the

recover, yet after 100 years the total

fossil fuels injects carbon

widely-used C-ROADS climate policy

carbon stored in the forest will still

sequestered in geological reservoirs

model to explore the dynamic

remain below the initial level.

for millions of years into the

impact of biofuels on carbon

This example is typical. We

atmosphere, causing global

emissions and climate. The model is

examined a wide range of forests in

warming. In contrast, biofuels

fully documented and freely

the US, the largest supplier of pellets

recycle carbon from the atmosphere,

available. Our research is

to the UK. Carbon debt payback

so, people argue, there are no net

independent, funded neither by

times averaged 87 years after

emissions.

bioenergy producers, who support

clear-cut, and 63 years when forests

Unfortunately, science shows

wood use, nor by environmental

are thinned.

otherwise. Burning wood to produce

groups, some of which oppose it.

These payback times are

energy can actually worsen climate

Indeed, given the urgency and

optimistic: we assume all land

change, at least through the year

magnitude of emissions reductions

harvested for bioenergy recovers

2100 - even if wood displaces coal,

needed to limit global warming to

without re-harvest, unplanned

the most carbon-intensive fuel.

no more than 2°C, my sincere hope

logging or conversion to other uses,

Why, and how do we know?

was that wood would prove to be

and without damage from fire,

part of the climate solution.

insects, severe weather or other

Supply and regrowth

Unfortunately, our results

ecological disturbances that would

An example of clearcutting

First, some basic physics:

in North Carolina in 2017

indicate that wood worsens climate

harm carbon uptake or inject

• Carbon dioxide is the principal

change through the rest of this

greenhouse gases into the

Photo: Dogwood Alliance

Energy World | May 2018 29

Energy storage and renewables

fuel. Carbon dioxide emissions per

unit of primary energy from wood

are about the same as coal, but 30%

higher than fuel oil and 80% higher

than natural gas.

We assume that the decline in

coal use resulting from wood does

not lower coal prices, and increases

coal demand elsewhere. Such

rebound coal demand would mean

the carbon debt from burning wood

would never be repaid, permanently

worsening global warming. Finally,

we don’t consider ecological

damage from harvesting wood,

including erosion and soil loss,

habitat fragmentation and resulting

wildlife loss, or declines in hunting,

recreation and tourism.

Of course, these results do not

support continued coal use. To have

any decent chance of limiting global

warming to no more than 2°C,

global greenhouse gas emissions

must start to decline in the next few

Figure 1. Simulation showing the recovery of an oak-hickory forest in the southern US after clear-cut - assuming the

land is not converted to pasture, agriculture, or other uses, not re-harvested, and suffers no fire, insect damage or other

years. Solar and wind with storage,

events that could harm regrowth

and especially energy efficiency, are

Source: John Sterman, Lori Siegel, Juliette Rooney Varga, bit.ly/forestbioenergy

the cheapest, safest, and quickest

ways to cut emissions while

providing the goods and services

atmosphere. It is worth noting that

released when they are burned.

people need. These technologies

climate change worsens fire, insect,

Could this be mitigated if forests

have a far smaller footprint than

and severe weather risks.

are managed sustainably, by

forest bioenergy and immediately

thinning instead of clear-cut? Large

lower emissions, without the need

Taking out more credit

pellet users and suppliers claim that

to wait for uncertain regrowth of

The International Energy Agency

they never cause deforestation and

forest lands.

and the bioenergy industry project

source low-grade wood including

Government policies should not

substantial growth in wood use for

tree tops, branches, and thinnings

violate basic laws of physics.

decades to come. Such expansion

(small trees). These claims are

Declaring that wood biofuels are

would steadily add to atmospheric

disputed.

carbon neutral, as the EU, UK and

carbon dioxide, even if the

But analysis indicates that wood

others have done, assumes regrowth

harvested lands fully recover.

bioenergy worsens climate change

is rapid and certain. Neither is true.

To see why, consider the national

even if never sourced by

This accounting fiction promotes

debt of the United States. The US

clearcutting. We examined scenarios

costly policies that accelerate

repays the bonds it issues in full at

in which all wood bioenergy is

climate change. Through renewable

maturity. However, because the

harvested by thinning. Although

energy subsidies, the UK and Europe

government runs a deficit, new

less damaging than clear-cut, the

are paying power plants to make

borrowing exceeds repayment of

average carbon debt payback time

climate change worse.

maturing debt, increasing the

for the US forests we examined was

Proper accounting would tally

national debt. Similarly, every year

still 63 years, and the resulting

the emissions from all sources of

wood biofuel use grows, the carbon

climate damage persists for

energy, whether coal, gas, solar or

dioxide added to the atmosphere

centuries even if the carbon debt

wood. For wood and other biofuels,

when that wood is burned exceeds

from thinning is fully repaid.

offsetting reductions in atmospheric

the carbon removed by regrowth,

What if wood pellets are sourced

carbon dioxide should be credited

increasing atmospheric carbon

from fast-growing managed tree

only when and if there is net new

dioxide even if all carbon debt is

farms instead of slow-growing

growth on the lands harvested to

eventually repaid in full.

natural forests? Counter to

supply the biomass.

Critically, during the decades

intuition, harvesting existing forests

Our results

Instead of subsidies that harm

before the carbon debt is repaid, the

and replanting with fast-growing

indicate that

the climate, a market-based

additional carbon dioxide in the

species in managed plantations

approach combining accurate

wood worsens

atmosphere from wood bioenergy

worsens the climate impact of wood

accounting with a meaningful price

worsens climate change. Higher

biofuel. The carbon density of

climate change

on carbon and other greenhouse gas

carbon dioxide concentrations

managed plantations is lower than

through the rest

emissions - no matter where and

resulting from wood use then

unmanaged forests, so the carbon

of this century at

how they arise - would lead to far

accelerates global warming, raising

sequestered in plantations never

larger emissions cuts at far lower

least - even

sea level faster and intensifying

offsets the carbon taken from the

cost. l

other damages including ocean

original forest. Converting forests to

when wood

acidification, the incidence of

plantations permanently worsens

displaces coal,

John D Sterman is the Jay W Forrester

extreme weather, water stress and

climate change.

Professor of Management at the MIT

the most carbon-

crop yield decline.

Sloan School of Management, Cambridge,

intensive fuel

These impacts persist for

Accounting fiction

Massachusetts, US, and Director of the MIT

System Dynamics Group. The paper on the

centuries or more. They are not

The assumptions of our study

above research, published in Environmental

reversed even if the forests

favour bioenergy. Specifically, we

Research Letters, is available at

harvested for bioenergy eventually

assume wood is used to offset coal,

bit.ly/forestbioenergy

take up all the carbon dioxide

the most carbon-intensive fossil

Energy World | May 2018 31